Add texture validation

[S-S-X]

Simple validation for textures, watches for node registration and checks if used textures exist.

To do:

• Default excludes for core textures.
• Proper texture definition parsing.
• Good enough report for failed texture, it should be easy to find problem source lines based on report contents.
• Move functionality away from core.lua, make it be new module that gets loaded based on configuration.
  • Consider multiple overrides for core registration functions, those might not be needed depending on how failures are handled.
• Currently just prints error messages, allow failing tests after registrations are done:
  • Possibly after on_mods_loaded is executed.
  • Or after all tests have completed, this ensures that all registered nodes can be checked.
• Allow handling node overrides by only looking for textures after tests have been executed.
  • Most probably it indicates problems if tests fail during registration but pass afterwards, probably not good.